1. Field of the Invention
The present invention relates to a vehicle suspension system and, in particular, to an active vehicle suspension system. More specifically, the present invention pertains to an active vehicle suspension system which provides improved ride qualities as well as improved rolling and pitching characteristics.
2. Description of Prior Art
Typical prior art vehicle suspension systems comprise a suspension spring and a damper. This type of suspension exerts suspension forces in response to external force acting thereon. Thus, the suspension has a specified spring constant and damping characteristics to thereby perform a desired oscillation under application of external force. It is known that the ride qualities of the suspension can be improved by reducing the spring and damping forces, whereas the rolling and pitching can be improved by increasing these forces. Therefore, proposals have been made to change the suspension characteristics according to driving conditions. For example, the suspension characteristics can be shifted to the hard side by increasing the spring and damping forces during cornering in order to reduce rolling of the vehicle body. However, where the curved road is rough, the suspension set to hard results in bad ride qualities. Thus, this type of suspension is limited in its ability to maintain both good ride qualities and good rolling and pitching properties.
Different types of suspensions have been proposed which comprise a suspension force exerting mechanism such as a hydraulic cylinder between the wheel and the vehicle body. These suspensions can exert different suspension forces irrespective of external forces acting thereon and are therefore called active suspensions. This type of suspension is, for example, disclosed in EPC application No. 0114757 wherein a hydraulic cylinder is provided between a wheel and a vehicle body and is controllably supplied with oil to thereby exert desired suspension forces and thus realize a preferable spring constant and preferable damping properties of the suspension. However, no suggestion is found in this patent about utilizing the active suspension to maintain both the ride qualities and the rolling and pitching properties in a preferable state throughout all types of running condition. An active suspension is also disclosed in Japanese patent publication No. 59-14365, wherein the active susupension is utilized to adjust the height of the vehicle body from the ground to a desired value, but not to improve the ride qualities and rolling and pitching properties of the suspension. Accordingly, a need still exists for improving both the ride qualities and rolling and pitching properties of a vehicle suspension system throughout all running conditions.